A magnetic tunnel junction is a device comprised of two ferromagnetic electrodes separated by a thin insulating layer and based on the phenomenon of spin-polarized electron tunneling. One of the ferromagnetic electrodes has a higher coercivity than the other. The insulating layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic electrodes. The tunneling phenomenon is electron spin dependent, making the magnetic response of the junction a function of the relative orientations and spin polarizations of the two electrodes. FIG. 1 shows a prior art MTJ device with Co and Co--Fe layers separated by an alumina (Al.sub.2 O.sub.3) insulating tunneling layer. FIG. 2 illustrates a typical result for the dependence of the junction resistance on the applied magnetic field. The magnetoresistance (.DELTA.R/R) response is hysteretic with a peak of maximum resistance occurring as the field is swept from a substantial value (e.g., 10-200 Oe) in one direction to a substantial value in the opposite direction. Near the middle of the sweep the resistance is a maximum when the magnetization vectors of the two electrodes point in substantially opposite directions, as indicated by the arrows above the magnetoresistance curve.
Although the possibility of applications for MTJ devices involving tunneling between ferromagnets has long been recognized, serious interest has lagged because of difficulties in achieving responses of the magnitude predicted in practical structures and at noncryogenic temperatures.
Prior to the present invention as described below, there have been no demonstrations of working MTJ devices with usefully large magnetoresistance responses (e.g., on the order of 10%) at room temperature in practical microelectronic device configurations. Experimental results for tunneling between ferromagnets were reviewed by R. Meservey et al. in "Spin-polarized Electron Tunneling", Physics Reports, Vol. 238, pp. 214-217, and showed only very small responses at room temperature, at best being on the order of 1-2%. The only indications of reasonably-sized responses were from two experiments with scanning tunneling microscopes. One of these employed a 100% spin-polarized CrO.sub.2 tip and indicated a polarized current modulation of 40% at room temperature, as described by R. Wiesendanger et al. in "Observation of Vacuum Tunneling of Spin-polarized Electrons with the Scanning Tunneling Microscope", Physics Review Letters, Vol. 65, page 247 (1990).
A very large MTJ device with an 18% magnetoresistance response was reported by T. Miyazaki et al. in "Giant Magnetic Tunneling Effect in Fe/Al.sub.2 O.sub.3 /Fe Junction", Journal of Magnetism and Magnetic Materials, Vol. 139, No. L231 (1995). However, the authors report that they could not reproduce their 18% magnetoresistance result. Other junctions fabricated at the same time had responses of only 1-6%.
Others have reported MTJ devices with magnetoresistance of up to 18% at room temperature in large Co--Fe/Al.sub.2 O.sub.3 /Co junctions, as described by J. S. Moodera et al. in "Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions", Physics Review Letters, Vol. 74, page 3273 (1995). However, these devices were formed by complex methods, including evaporation onto cryogenically-cooled substrates. The junction resistances were in the range of hundreds of Ohms to tens of kOhms for junctions with large cross-sectional areas of 200.times.300 .mu.m.sup.2.
Thus, it is apparent that it has been difficult to make MTJ devices at room temperature with a large enough magnetoresistance response to be useful. The first observation of a magnetoresistance response of the expected magnitude at room temperature occurred in a spin-polarized scanning tunneling microscope. Subsequently in the prior art, MTJ responses of the expected magnitude at room temperature have been obtained, but only for large devices made using exotic and impractical thin film deposition techniques. It has not yet been demonstrated how to achieve a large magnetoresistance response in a practical microelectronic device configuration.
An additional problem with prior art MTJ devices is that the magnetoresistance response versus magnetic field has the characteristic two-hump shape response illustrated in FIG. 2. A step-like magnetoresistance response has been demonstrated over a restricted applied magnetic field range, as described by T. Miyazaki et al. in "Large Magnetoresistance Effect in 82Ni--Fe/Al--Al2O3/Co Magnetic Tunneling Junction", Journal of Magnetism and Magnetic Materials, Vol. 98, No. L7 (1991). However, if the applied magnetic field excursion were momentarily too large, the magnetoresistance response characteristic could be inverted, as shown in FIGS. 3A-3B.
What is needed is an MTJ device with unambignous and controlled magnetoresistance response to magnetic signals that can be mass fabricated and can be scaled down in size to deep submicron dimensions.